There are 13.5 million children of immigrants in the U.S., which accounts for nearly one-fifth of our nation's children and a disproportionate share of our nation's children in poverty. These children are at greater risk to drop out of high school and therefore realize worse economic outcomes. The schooling gap between the children of immigrants and native Whites appears to be linked to English proficiency. Unfortunately, since language human capital is endogenous, simple correlations of English proficiency and socioeconomic outcomes are unlikely to measure the causal effect of English proficiency. We take advantage of the psychobiological phenomenon that younger children learn languages more easily than older children to construct an instrumental variable for language human capital. We use instrumental-variables estimation to examine two main research questions. First, what is the effect of parents' English-language skills on children's language and educational outcomes? Second, to better understand the family environment in which the children of immigrants are growing up, what is the effect of English-language skills on the marriage, childbearing and location decisions of immigrants? We employ micro-data from the 2000 Census. Preliminary analysis reveals a powerful association between age at arrival and the English proficiency of individuals who immigrated during childhood. Regarding the first question, we find that immigrant parents' English proficiency has a positive effect on U.S.-born children's English proficiency while the children are still young. Also, children with parents with lower English proficiency are more likely to drop out of high school and not attend preschool. Regarding the second question, we find that higher English proficiency has no impact on the probability of ever marrying but raises the probability of divorce. Also, it does not change the probability of having at least one child but raises the number of children. The significance of the proposed research is that it will increase our understanding of the role of English in immigrant assimilation in the U.S. Knowledge gained will help us design policies that improve the conditions under which the children of immigrants grow up and that raise their well-being in adulthood. Psychobiological constraints on how humans acquire new languages may be preventing some immigrants from developing adequate English skills. Interventions can prevent the parents' disadvantages from being transmitted to the children. [unreadable] [unreadable]